Cutter-suction dredgers (CSD) are known. Such dredgers use a suction tube which has a cutter head at the suction inlet. The cutter head may be connected to the dredger with a hub that is mounted on an axis with a drive to rotate the cutter head. The axis of rotation is referred to as the axial direction of the cutter head. The material cut by the cutter is sucked into the suction tube and transported away from the cutter head, for instance via a floating pipe line to a dumping location.
The cutter head cuts and loosens the bed material such that it can be sucked into the suction tube. The cutter head and suction inlet may be moveable with respect to the water bed.
In order to suck the bed material into the suction tube a wear-resistant pump may be provided, such as a centrifugal pump.
Cutter-suction dredgers are often used to cut hard surface materials, such as rock, although they may also be used to excavate gravel or sand.
The cutter head may be provided with a plurality of excavating tools, such as teeth, formed as chisels, to chisel the bed material. The excavating tools may also be formed by a cutting edge comprising a plurality of teeth. However, the excavating tools are prone to wear, especially when hard surface materials are cut.
Therefore, cutter heads are known which comprise replaceable teeth or replaceable cutting edges. Replacing teeth is a time and therefore money consuming operation and contributes to the down-time of the cutter-suction dredger.
Known cutter head designs comprise a base ring provided around the suction opening to which a plurality of arms are connected. The arms extend in an axial direction away from the suction opening and converge towards each other radially thereby forming the cutter head in front of the suction opening. The arms may be curved in a tangential and radial direction such that the arms spiral towards each other. The arms may also be curved in the radial direction only, while being axially aligned or at a small angle with respect to the axial direction. Such a design is for instance known as the Lancelot (manufactured by IHC Parts & Services).
The converging arms may approach each other in the middle at a distance in front of the suction opening where they are connected to the hub which drives the cutter head.
The excavating tools, such as teeth or cutting edges are attached to the arms. The bed material is loosened and cut by the excavating tools and is sucked into the suction opening through the space in between the arms. The teeth also functions as scoops, which scoop the cut and/or loosened material from the water bed, scooping it towards the suction opening.
Known cutter heads have a number of disadvantages.
For instance, the teeth, formed as chisels, are prone to wear and need to be replaced often, increasing the downtime of the cutter head. In some situations, the teeth do not last more than an hour.
U.S. Pat. No. 3,885,330 and EP-A1-0376433 show an apparatus for digging a hole and a cutting tool which are not suitable for use in combination with cutter-suction dredgers. These documents show tools with arms comprising a plurality of excavating tools positioned in a plane perpendicular to an axis of rotation of the tool. Such tools will therefore not be suitable to function when positioned at an angle other than perpendicular with respect to the water bed. Also, such cutting tools are not suitable for being moved in a direction perpendicular to their rotational axis, as is common in cutter-suction dredgers, wherein the cutter-suction dredgers are provided with spud systems which allow the cutter-suction dredger to perform a rotating movement with respect to a spud, thereby moving the cutter head along the water bed.
WO2011003869 discloses such a cutter head for dredging ground under water. This cutter head is suitable for attachment rotatably around a central axis to the ladder of a cutter suction dredger and for being moved through the ground therewith in a lateral sweeping movement. The cutter head comprises a base ring, a hub situated at a distance in the direction of the central axis from the base ring, and a plurality of support arms extending from the base ring to the hub, wherein a passage opening is located between support arms and wherein the cutting tools are axisymmetrical, at least at their free outer end. The cutting tools are axisymmetrical at least at their free outer end, and preferably along their entire length, thereby allowing free rotation around their longitudinal axis. The cutting tools may be rotation-symmetrical and preferably of conical form. Such cutting tools take up less space making it possible to provide the cutter head with a large number of cutting tools. The cutting tools may be arranged in a socket such that it can be rotated freely or at least readily around its axis of rotation-symmetry. Allowing free or ready rotation of the tools during operation reduces the risk for breakage and also self-sharpens the soil-contacting tip of the cutting tools by friction with the soil.
In a preferred embodiment the cutter head according to the invention comprises at least 50 cutting tools, more preferably at least 100 cutting tools, still more preferably at least 140 cutting tools, and most preferably at least 180 cutting tools. The cutting tools can here be distributed regularly, but also irregularly, over the revolving surface of the cutter head. The number of cutting tools per support arm preferably comprises at least 10 cutting tools, more preferably at least 15 cutting tools, still more preferably at least 20 cutting tools, and most preferably at least 25 cutting tools.
However, in the prior art there is a prejudice against the use of such cutting tools which are axisymmetrical at least at their free outer end. Reference is also made to WEAR RESISTANT DREDGE CUTTER TEETH A LOOK AT THE DEVELOPMENT OF THE TOOTH AND ITS IMPACT ON THE ECONOMICAL AND ENVIRONMENTAL ASPECTS OF THE DREDGER LOGISTICS AND FOUNDRY, by Klaas Wijma, in Proceedings of the CEDA dredging days 2009, Dredging tools for the future, Rotterdam (www.dredging.org). According to this article, tests have been done with bits on conventional dredge cutter heads without success. A number of reasons for the lack of success are identified by Wijma. In the first place, the useful length of the hard metal rod in the conical bit is relatively short, resulting in a short life time. Secondly, the conical bits are designed for small cutting depths, resulting in a low production. Thirdly, the strength of the bits is relatively small, approximately 150 kN, where teeth can withstand cutting forces or normal forces in the range of 1500-2000 kN. All these disadvantages result in fast wear of the bits and the adapter (holding the bit), higher torque and penetration forces and increased breakages.
JP-U-50038142 shows the use of non rotational symmetric bit ends. The device according to this document has the disadvantage that it will not be effective in cutting material at the distal end of the cutter head, as no excavating tools are present there.